Fire in the Hole

The August Life of a Spark Plug

Feature Article from Hemmings Classic Car

In an effort to circumvent a host of vitriolic letters over the fact that an apparatus with no moving parts is being featured as a Mechanical Marvel, we offer this simple but effective defense: Without this gem of an invention, no gasoline engine would ever run. For this reason alone, the spark plug is worthy of analysis.
The function of the spark plug is to conduct the high potential energy from the ignition system into the combustion chamber and then to allow its discharge across an air gap.
The design and material used to construct the spark are very critical to its performance or lack of it. The electrode material needs to permit the formation of a spark discharge at a low and constant voltage; it should have good thermal conductivity; and it needs to withstand high temperatures, corrosive gases and erosive current discharges. The operating temperature range alone that a spark plug must endure is remarkable. Consider that on a cold start, the plug is at or very near ambient temperature, and on the first successful ignition event is exposed to a flame front with a leading-edge temperature of approximately 1,500 degrees F. To accomplish the desired results, a spark plug is constructed of platinum, or more commonly, nickel-chromium-barium alloys.
Early spark plugs faced the problem of establishing an arc across the electrodes, and research found that not only the potential energy in the ignition system impacted this but the characteristic of the plug itself played an important role. Spark plug pioneers determined that the arc was impacted by the following factors:
1. The length of the gap. The larger the gap between the center and side electrode, the more coil energy required.
2. The geometry of the gap. Pointed (small) electrodes require less voltage. Thus, the surface condition of the electrodes is important.
3. The temperature of the electrodes and enclosed air/fuel mixture. High temperatures allow lower voltage requirements.
4. The density of the mixture. High density mixtures created by large throttle openings required more energy due to higher cylinder pressure.
5. The leakage resistance of the insulator. Carbon and metallic oxides form electrically conductive coatings on the insulator, which thus shunt the secondary winding and reduce the maximum voltage that the secondary can provide across the spark plug gap.
6. The rate of increase of the voltage at the gap. If the ignition system builds up the voltage at a rapid rate (high frequency), the effect of leakage will be minimized, and a greater arcing voltage is available. 7. The presence of ionized gases in the gap.
8. The air/fuel ratio determines the electrical properties of the mixture. Lean mixtures have higher arcing requirements than rich mixtures.
9. The electrode material.
Because of the large number of variables it was difficult for early development engineers to evaluate a change in one factor; unknown changes in other variables were usually present and, therefore, an increased gap, for example, may not require a higher breakdown or ionization voltage.
It cannot be assumed just because an arc is created in the cylinder that the mixture will ignite. Good ignition depends upon the following variables:
1. A combustible mixture must be present between the electrodes.
2. A large gap increases the probability of a proper firing event, especially at light load when stratification from exhaust gas dilution is present.
3. A high mixture density allows a greater amount of energy to be liberated, and the probability of ignition is increased.
4. Ignition is best initiated with a slightly rich mixture since a greater release of energy is realized.
5. The position of the plug and the position of the electrode to the flow conditions of the combustion chamber.
The spark plug is screwed into the cylinder head with a protrusion into the combustion chamber. When the piston is at the correct position to accept ignition, a high-voltage current of electricity is sent along the metal center rod known as the electrode of the spark plug and across a small air gap. Although the gap is application-specific to the design and the engine, it is approximately 1/64-inch to 1/32-inch wide. The air gap creates such a great resistance to the current flow that it requires between 5,000 and 10,000 volts of electricity to bridge this distance.
The making of a spark plug
In the early days of spark plugs there were two main designs: separable and the integral or one-piece. Separable designs were just as their name implied and allowed the insulator or core to be removed from the main body. The one-piece design did not allow disassembly and is what we accept as a common spark plug today.
The components that make up a spark plug are the iron shell, or body, which screws into the cylinder head; the insulation which is held in the shell by the bushing or gland nut; the center electrode, which passes through the insulation and carries the electrical current to the gap. In addition, there is a side electrode that is moored to the body, and in some applications, washers or gaskets are used to keep combustion gases and pressure in the cylinder bore. If a washer/gasket is not employed, then a taper is machined into the body seat.
Early plugs used an insulation that was made from mica, but due to problems created by washers that would absorb oil between the layers and create a misfire, their days were limited. However, mica plugs were retained in aircraft engines through the 1940s since the material had excellent heat-resistance capabilities. The insulation that was made popular and is still in use today is porcelain, but its fragility makes it important to handle the plugs with care to avoid cracking and, thus, electrical leakage.
The electrode is usually made from steel or manganese nickel alloy, or a special alloy specified by the manufacturer. It is important for the metallurgy to be correct or the electrode will expand under the extreme heat of combustion and fracture the porcelain insulator. Typically, the electrode of a spark plug is 0.050- to 0.060-inch in diameter.
A cement is used to seal and bond the insulator to the body and retain the electrode in place. As the cement dries, it becomes porous, and this can result in electrical leaks. For this reason, spark-plug designers must concentrate on the dual leakages of both the gaseous mixture and electrical current. Poor engine performance can frequently be traced back to a poorly designed or manufactured spark plug.
Likewise, if the spark plug porcelain becomes porous, it will provide an electrical path to ground through the threads and cylinder head, creating a dead or misfiring cylinder.
As with many components, as they evolved, early spark plugs were made in many different shapes, sizes and designs. It was common to see plugs of unique threads, lengths and shapes. As a matter of fact, a large hobby exists in collecting different spark plugs. The most common designs in the early days fell into three standard threads: the 1/2-inch pipe thread; metric; and 7/8-inch 18 SAE.
The 1/2-inch size was a thread which is a standard 1/2-inch iron pipe size but with a slight taper.
The 7/8-inch 18-size was the one first adapted as a standard by the Society of Automobile Engineers (SAE) for automobile use. It used to be known as the A.L.A.M. The thread is 7/8-inch in diameter, with 18 threads to the inch. It was used on a majority of early engines until the 14-mm design was accepted as a standard.
The metric size was smaller than either of the others. Its diameter was 18 millimeters or approximately 23/32-inch; pitch: 1.5 millimeters. This eventually became the thread standard by SAE for car and aircraft engines. It was also used on many motorcycle engines.
Early standards by SAE included two spark plug shells. This described the diameter of the hexagon part of the shell and would identify what size wrench or socket that would attach for servicing. At that time in history (the 1930s and earlier), a spark plug was either a 15/16-inch or a huge 1.125-inch design. The previously mentioned 7/8-inch plug required a special tap cutting of 18 threads, whereas the standard 7/8-inch thread tap was 14 threads to the inch. Eventually new, more modern designs surfaced with 13/16- and 5/8-inch shells utilizing both 14-mm and 18-mm thread design.
The length of a spark plug depends on the engine design. If the valves were deeply recessed, a long body plug was required; otherwise it would be difficult to remove for service, since a special wrench would be needed. If they are not recessed, a long thread is required.
The length from the tip of the electrode to the engine side of the shell or body is called the reach. Many engine designers mandate a special plug reach to obtain the most performance from their powerplant.
In the early days of spark-plug design, it was thought the most beneficial placement of the electrode in the bore was to have a bias toward the intake valve. The logic was based on a greater amount of combustible mixture that could be located in the gap and aid the ionization event. During that time, it was also believed that a center spark plug location was the most undesirable, and any trajectory toward the exhaust valve would be prone to abnormal combustion from the excessive heat in that region. Any enthusiast who has taken older engines apart can attest to this. Pontiac, as late as the 1960s, still followed this logic, producing many cylinder heads with intake port-biased spark plugs. As engine development advanced, it was found that the early discoveries were completely backward from what is now understood. An idealized cylinder head has the spark plug located in the center of the bore-as does a true hemi-which is the most turbulent and hottest region. This allows for a quicker burn rate and established reaction zone. During the early stages of combustion, a reaction zone needs to be created where the heat and pressure from the burned mixture is transferred into the unburned charge, thus increasing the rate of expansion across the bore. The completely incorrect design protocols of early spark plugs were the direct result of ignition coil and combustion chamber design inexperience, not of faulty engineering. The spark plug and its placement were unknowingly being manipulated to make up for deficiencies in other areas of engine development.
Today's automotive enthusiast is once again living through a dynamic time in spark-plug design as advanced combustion-chamber theory and completely new ignition logic is being employed. One has to look no further than the advertisement for Ford's new F-150 truck and the unique, extremely long spark plug with a U-shaped side electrode that bridges over the center electrode that it uses. In addition, the extremely high voltage and low current that today's ignition systems work on along with the use of platinum center and side electrodes means many mechanics and car owners hardly ever see the spark plugs. No longer does one spend a few hours every couple of thousand miles removing, cleaning and re-gapping spark plugs as we once did. With today's spark plugs requiring no service for 100,000-plus miles, many cars will go to the junkyard with the factory plugs never removed. The joy of spark plug servicing and the window into the combustion event that it provided are a part of motoring history that I and many of our readers will miss. Good-bye old friend.

This article originally appeared in the March, 2005 issue of Hemmings Classic Car.